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Synthesis of One-Dimensional Carbon Nanomaterials Wrapped by Silver Nanoparticles and Their Antibacterial Behavior Aping Niu,† Yujie Han,‡ Jian Wu,‡ Ning Yu,† and Qun Xu*,† College of Materials Science and Engineering and Department of Bioengineering, Zhengzhou UniVersity, Zhengzhou 450052, China ReceiVed: May 23, 2010; ReVised Manuscript ReceiVed: June 22, 2010

One-dimensional (1D) carbon nanomaterials wrapped by silver nanoparticles were fabricated via a facile and environmentally benign route with the assistance of supercritical carbon dioxide. Transmission electron microscopy, scanning electron microscopy, and energy-dispersive X-ray analysis revealed that carbon nanofibers (CNFs) were densely coated by silver nanoparticles under the optimized experimental condition. In the case of carbon nanotube/silver (CNT/Ag) nanohybrids, these silver nanoparticles on the surface of carbon nanotubes were predominantly spherical in shape with excellent dispersion, and their sizes were smaller than that on carbon nanofibers. The UV-vis spectra presented a surface plasmon resonance vibration band at 448 and 414 nm for CNFs and CNTs, respectively. X-ray diffraction analysis showed that the nanoparticles were of a face centered cubic structure. Some crucial factors, which affect the growing and arraying of Ag nanoparticles along the axis of 1D carbon nanomaterials, had been investigated. As examples for promising applications, the antibacterial activities of the as-prepared one-dimensional nanocomposites were also studied. 1. Introduction As one of the members of the noble metallic nanomaterials family, silver nanoparticles (AgNPs) have sparked intense excitement in nanotechnology and biotechnology due to their high catalytic activity and excellent antimicrobial activity.1 For instance, they can be used as electrocatalysts for fuel cells,2 for the reduction of organic halides,3 in biolabel materials,4 and in antibacterial materials.5-9 Recently, because of increasing attention on preventing diseases in public health hygiene,10-13 a great deal of effort has been ploughed into the biosynthesis of antimicrobial silver materials. Some antimicrobial composites have been investigated, such as poly(methyl methacrylate) (PMMA) nanofiber containing AgNPs,7 silver/poly(ethylene oxide) (PEO) nanocomposites, and silver/polyrhodanine nanofibers.8,9 Meanwhile, some biological systems (bacteria, fungi, and algae) used for AgNP synthesis have also been reported.14-16 Many protocols are used to synthesize AgNPs, involving a template method,17,18 chemical reduction,19,20 and photochemical reduction.21 Conventionally, the reduction of silver ions (Ag+) is obtained by chemical methods. The biological toxicity and the environmental hazard of some residual reducing agent are inevitable, such as NaBH4,22 formamide,23 dimethylformamide, and hydrazine.23,24 Therefore, exploring a simple, fast, and environmentally friendly method to synthesize silver nanoparticles is necessary. Carbon nanofibers (CNFs) have inherent advantages, such as excellent chemical stability and mechanical stability. As a substrate for an antimicrobial system, its nanofibrous structure can offer higher cell adherence compared with other structures.25-27 Besides, considering their other functional properties, they have been used for the fillers in polymer composites, or used as the biosensors.28,29 As another 1D carbon material, carbon nanotubes * To whom correspondence should be addressed. E-mail: qunxu@ zzu.edu.cn. Tel: +86 371 67767827. Fax: +86 371 67767827. † College of Materials Science and Engineering. ‡ Department of Bioengineering.

(CNTs) have attracted more interest in various fields, including chemical and biological sensors,30,31 separation membranes,32 energy storage,33 and improved accessibility of reactants to the active sites.34 It has been confirmed by calculations and experiments that 1D metal NPs can show a greater local field enhancement,35,36 and the axial ratio for the 1D NPs has a special effect.37 Considering that CNTs and CNFs belong to 1D nanomaterials, they can evidently improve the physical properties of metal NPs.38 It is anticipated that the composite of 1D carbon materials and zero-dimensional Ag nanoparticles can contribute to antimicrobial efficiency. Supercritical fluid (SCF) technology has become an important tool for materials processing.38,39 Among SCFs, supercritical carbon dioxide (SC CO2) is extensively employed. As a green solvent, SC CO2 is a substitute for conventional organic solvents because it is nontoxic, nonflammable, inexpensive, naturally abundant, and environmentally benign. Its low viscosity, high diffusivity, and negligible surface tension play important roles in preparing superior products of fine and uniform particles.40 These unique properties of SC CO2 make it an attractive medium for delivering solutes to small areas with complicated surfaces and poorly wettable substrates to attain a high uniformity and homogeneity.41,42 In addition, the high solubility of SC CO2 in many organic solvents leads to a decrease of the solvent strength. This antisolvent effect has been used to construct nanomaterials, such as Eu2O3-coated CNTs,43 platinum/CNT nanocomposites,44,45 metal nanowire-filled CNT nanocomposites, etc.46 Considering the unique characteristics of AgNPs, 1D carbon materials (CNFs and CNTs), and SC CO2, it intrigues us to study the fabrication of 1D carbon nanomaterials wrapped by AgNPs with the assistance of SC CO2. In this study, we choose glucose, a commonly available and nontoxic chemical, as the reducing agent to take the place of the traditional agent. Silver nitrate (AgNO3) was used as the salt precursor. The 1D carbon materials/Ag nanohybrids were obtained in an ethanol/CO2 system. We studied the effects of the various pressures and temperatures of SC CO2 on the AgNPs wrapping around CNFs.

10.1021/jp104720w  2010 American Chemical Society Published on Web 07/06/2010

1D Carbon Nanomaterials Wrapped by Ag Nanoparticles

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Figure 1. (A) TEM images of a pristine CNF and (B) a AgNP-decorated CNF prepared in the experimental condition of 12 MPa and 65 °C for 3 h. (C) Typical XRD pattern of the CNF/Ag nanocomposites fabricated under 12 MPa/65 °C.

It can be found that AgNPs adsorbed on CNTs presented a regular shape and excellent dispersion compared with Ag deposited on CNFs, so the different formation mechanism of AgNPs and their assembly on CNFs and CNTs is studied. Further, the antibacterial activities of the obtained 1D carbon materials/Ag nanohybrids against E. coli are also studied. 2. Experimental Section 2.1. Materials. Silver nitrate (AgNO3) was purchased from Tianjin Kermel Chemical Reagent Co. Ltd., and anhydrous ethanol (C2H5OH) was offered by Anhui Ante Biochemical Company, Ltd. Anhydrous glucose was of analytical grade and used as a reducing agent. All the chemicals were used without further purification. Carbon nanofibers (CNFs) and carbon nanotubes (CNTs) were purchased from Pyrograf Products, Inc. (U.S.A.) and Timesnano without any pretreatment. CO2 with a purity of 99.9% was provided by Zhengzhou Gas Company and used as received. 2.2. Fabrication of 1D Carbon Nanomaterials Wrapped by Ag Nanoparticles. AgNPs wrapping around 1D carbon materials were obtained by the following experimental process: carbon materials were dispersed in 2 mL of ethanol solution and ultrasonicated for 2-3 h at room temperature to form a homogeneous solution. AgNO3 used as a silver precursor and

glucose used as a reducer were dissolved in 2 mL of ethanol. After a few minutes, the AgNO3 solution was added into the ethanol solution. Consecutively, the mixture was quickly transferred into a 50 mL stainless steel autoclave. CO2 was then charged into the autoclave, adjusting the temperature and pressure of the autoclave to the desired value and keeping the SC CO2 condition for 3 h under magnetic stirring. The CO2 was then vented slowly, and the sample was collected and labeled. 2.3. Characterization. Transmission electron microscopy (FEI Tecnai G2 20) was used to characterize the morphology of AgNPs wrapping around carbon materials, which were conducted with an accelerating voltage of 120 kV. The optical properties have been studied using UV-visible spectroscopy. X-ray diffraction patterns of the products were recorded on a Rigaku D/MAX-3B using Cu KR radiation at a scanning speed of 15.24° (2θ)/min in the range of 20-85°. The surface morphology of CNF/Ag nanocomposites was also studied by a scanning electron microscope (FEI QUANTA-200). 2.4. Test of Antibacterial Activity. The antibacterial properties of the as-prepared 1D carbon materials wrapped by AgNPs against Escherichia coli were tested using the spread plates and optical density (OD), which was measured using a UV-vis spectrophotometer at 600 nm. E. coli was cultivated in a

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Niu et al. suspensions. The nanohybrid suspensions with different volumes (1 mL (1%), 0.5 mL (0.5%), 100 µL (0.1%), 50 µL (0.05%)) were then added into 100 mL of the nutrient broth with 5 mL of the E. coli suspension. The nutrient broth solution of bacterial suspensions without any nanohybrids added was made as the control. Subsequently, the resultant solutions were inoculated at 37 °C with a shaking incubator. The concentration of the bacterial suspensions was determined by spectrophotometry. Meanwhile, 100 µL suspensions of bacteria were spread on each LB agar plate and incubated overnight in the dark at 37 °C.

Figure 2. UV-vis spectra for the CNF/Ag nanocomposites fabricated at 12 MPa and 65 °C for 3 h and pure AgNPs obtained at 65 °C for 3 h.

sterilized nutrient broth and then incubated overnight at 37 °C with a shaking incubator. During the test with optical density and Luria-Bertani (LB) agar plates, the as-prepared nanohybrids were uniformly distributed in 4 mL of sterile water to form

3. Results and Discussion 3.1. Carbon Nanofibers Decorated by Ag Nanoparticles. Figure 1A shows the TEM image of a pristine single CNF. The diameter of the CNF is about 80 nm. It can be observed that the wall of the CNF is smooth and the center presents a cupshaped structure. Figure 1B shows the typical TEM image of CNF/Ag nanocomposites obtained under 12 MPa and 65 °C. By comparison, we can see that the sidewall of the CNF was densely wrapped with a silver nanoparticle layer, which shows that AgNPs could adsorb on the surface of the CNF. Meanwhile, the TEM image indicates that the size of AgNPs coated on CNFs is small and its average size is 8 nm. The formation of AgNPs on the outer surface of CNFs is confirmed by X-ray diffraction analysis (Figure 1C). The strong diffraction peaks at 2θ values

Figure 3. (A, B) Representative SEM images of the CNF/Ag nanocomposites gained at 12 MPa/65 °C. (S1, S2) Energy-dispersive X-ray (EDX) of the chosen sample regions in (B).


J. Phys. Chem. C, Vol. 114, No. 29, 2010 12731 SCHEME 1: Sketch of the Formation of CNF/Ag Nanocomposites in SC CO2 Conditions

Figure 4. TEM images of CNF/Ag nanocomposites obtained at different pressures with the same temperature (65 °C): (A) 10, (B) 11, and (C) 12 MPa.

of 37.46, 44.02, 64.22, and 77.12° can be indexed to diffractions from the (111), (200), (220), and (311) crystalline planes of face-centered cubic (fcc) silver (JCPDS Card File 4-783). UV-vis absorption spectra were carried out to characterize the optical absorbance of the synthesized CNF/Ag nanocomposites. For comparison, the directly reduced AgNO3 was used as the control experiment. Figure 2 shows the spectra of both samples fabricated at 12 MPa and 65 °C. It can be observed that CNF/ Ag nanocomposites and pure AgNPs have a maximum peak at 448 nm, which corresponds well to the silver plasmon,47 and CNF/Ag nanocomposites have a relatively strong and sharp absorption peak compared with pure AgNPs. To further confirm the experimental phenomenon, scanning electron microscopy (SEM) was employed to characterize the decorated CNFs. The representative SEM image presented in Figure 3A indicates that a great deal of AgNPs were attached and distributed along the CNFs. The energy-dispersive X-ray analysis (EDX) shows the signal of elemental silver in the chosen regions in Figure 3B, where an optical absorption band exists at 3 keV due to surface plasmon resonance.48 For the supercritical CO2-assisted reaction system, experimental pressure has an important effect on the solvent power. We study the effect of SC CO2 on AgNPs wrapping around CNTs by adjusting the experimental pressure from 10 to 12 MPa. Figure 4 shows the TEM images of the synthesized CNF/ Ag nanocomposites. The results indicate that, when the experimental pressure is 10 MPa, there is just aggregated AgNPs. As

the pressure increases to 11 MPa, the AgNPs distribute along the CNFs and their aggregation is weakened. When the pressure reaches 12 MPa, the CNFs are densely wrapped by AgNPs with smaller particle sizes. This phenomenon can be explained as the following (Scheme 1). At lower SC CO2 pressures, the AgNPs in solution are abundant and the collision probability is comparatively higher, which contributes to the aggregation of AgNPs. When the pressure is increased, more small silver crystals can be quickly deposited and homogeneously coat on the CNFs under stirring; therefore, the aggregation is avoided. Considering that CNF/Ag nanohybrids are induced by the SC CO2 antisolvent effect, it can be concluded that experimental pressure is a decisive factor for the AgNP coating on the CNFs. To optimize the reaction temperature, a temperature variation study was carried out at 12 MPa. TEM images of AgNPs obtained at different temperatures are presented in Figure 5. It can be observed that the AgNPs are well distributed, but their

Figure 5. TEM images of Ag-wrapped CNFs produced in the same SC CO2 conditions (12 MPa for 3 h), with different temperatures: (A) 60, (B) 65, and (C) 70 °C.

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Figure 7. UV-visible absorption spectra of CNF/Ag and SWCNT/ Ag nanocomposites synthesized at 12 MPa and 65 °C for 3 h.

Figure 6. TEM images of the SWCNT/Ag nanohybrids in the same SC CO2 conditions (12 MPa for 3 h), with different temperatures: (A) 60 and (B) 65 °C.

distribution density is relatively sparse at 60 °C. Although, when temperature rose to 65 °C, well-assembled CNF/Ag nanocomposites with a thicker density and the tiny size of the crystals could be obtained. When the temperature further rose to 70 °C, there are scarcely anu AgNPs on the CNFs. The results indicate that the experimental temperature is also an important factor on the preparation of CNF/Ag nanohybrids. In fact, the rate of the redox reaction depends on the temperature. At lower temperatures, it is slow and the reaction is inadequate. At higher temperatures, the reaction rate accelerates, and more silver ions are reduced. In addition, with the temperature increasing, the density of CO2 in the autoclave diminishes. Therefore, the amount of CO2 dissolved in ethanol decreases at the same pressure, which makes the degree of supersaturation become low, and ultimately, AgNPs precipitated from solution are reduced. Considering these factors, it can be supposed that the redox reaction rate and antisolvent effect of CO2 reach a balance at 65 °C, at which the well-decorated CNF/ Ag nanocomposites can be obtained. It can be concluded that the optimal condition for preparing CNF/Ag nanocomposites is 65 °C and 12 MPa. 3.2. Carbon Nanotubes Decorated by Ag Nanoparticles. We also conducted the parallel experiments of AgNPs wrapping around CNTs. First, single-walled carbon nanotubes (SWCNTs) were selected as a template to prepare CNT/Ag nanohybrids. Figure 6 shows a set of TEM images of SWCNT/Ag nanocomposites prepared at 60 and 65 °C. The diameter of the SWCNTs used in the experiment is 1-2 nm. It can be observed that the

Figure 8. Typical XRD pattern of the SWNT/Ag nanohybrids obtained in SC CO2 at 65 °C/12 MPa for 3 h.

surface of the SWCNT bundles is anchored with uniform and spherical nanoparticles. The average size of these particles measured from TEM images is about 4-6 nm, which is smaller than that on CNFs. The UV-vis spectra of AgNPs deposited on SWCNTs can be observed from Figure 7. It is found that AgNPs decorating on SWCNTs present a blue shift from 448 to 414 nm comparing with AgNPs on CNFs. The phenomenon of blue shift means that the AgNPs have a smaller size, which is also conformed from the TEM characterization.49 Figure 8 is the XRD pattern of SWCNT/Ag composites obtained at 12 MPa and 65 °C. The peaks at 37.9, 44.1, 64.8, 77.4, and 81.6° correspond to the strongest reflection of (111), (200), (220), (311), and (222) of the silver phase, respectively. The average particle size (Å) can be calculated from the broadening (111) reflection by using the Scherrer formula, D ) (Kλ)/(β cos θ), where D is the mean particle size in Å, λ is the wavelength of the X-ray (λ ) 0.154 nm), K is the Scherrer constant (K ) 0.9), θ is the angle of the peak maximum, and β is the full width at half-maximum (fwhm) of the (111) plane (in radians). The average size of AgNPs is calculated to be 5.0 nm, which is identical to the estimated values from the TEM images. Figure 9 shows the TEM of CNT/Ag nanocomposites prepared at different pressures. It can be observed that the shape of the AgNPs is more regular and their distribution is well


J. Phys. Chem. C, Vol. 114, No. 29, 2010 12733 SCHEME 2: Schematic Illustration for the Generation of CNF/Ag and CNT/Ag Nanohybrids with the Assistance of SC CO2

Figure 9. TEM images of SWCNT/Ag nanocomposites obtained at the same temperature (65 °C) and different pressures: (A) 10, (B) 11, and (C) 12 MPa.

distributed with the increasing of pressure. The effect of pressure on preparing CNT/Ag nanocomposites is in accord with that of CNF/Ag nanocomposites. However, the shape and size of AgNPs attached on CNTs are apparently better. Next, we will discuss the formation mechanism of the two 1D carbon materials as a template for Ag. 3.3. Formation Mechanism of AgNPs on Different SubstratessCNFs and CNTs. This work demonstrates the feasibility of wrapping 1D carbon materials with AgNPs via the reduction of metal nitrate precursors in an ethanol/CO2 system. In this work, SC CO2 played a pivotal role in decorating 1D carbon material with AgNPs. First, SC CO2 is miscible with ethanol at suitable conditions. When ethanol acts as a solvent for precursors, the zero surface tension of supercritical CO2 makes ethanol dampen the surface of carbon materials well during the whole experimental process. Consequently, SC CO2 helps the precursor easily absorb on the surface of carbon materials and enhances the physical attraction of the two substances. Second, SC CO2 may act as the antisolvent for the Ag nanoparticles in the expanded ethanol system. As the dissolution of CO2 increases, the solvent power of the liquid phase on AgNPs is decreased, which can lead to the phase separation, and AgNPs precipitate out of the supersaturated solution. The inherent properties of carbon materials as a substrate are a key factor for the size and morphologies of the formed AgNPs. Scheme 2 illustrates the synthetic mechanism of AgNPs on CNFs and CNTs. CNF/Ag composites were obtained by mechanism 1 in Scheme 2. Because the outer surface of CNFs is comparatively glazed (Figure 1A) and ethanol is a polar solvent, whereas CNFs are tantamount to a nonpolar solvent, the surface wetting of CNFs is difficult. A metal precursor cannot be deoxidized in situ on the surface of CNFs; it just can be deoxidized in solution. When the dispersed Ag+ was deoxidized to AgNPs, because of the dissolution of CO2 in solution, the solvent power of the liquid phase on AgNPs was decreased, which can lead to AgNPs precipitating out of the supersaturated solution and depositing on CNFs. The formation mechanism of CNT/Ag nanohybrids in an ethanol/CO2 system

can be illustrated by mechanism 2. It is well known that CNTs consist of rolled graphene sheets, and the surface of CNTs is special and has some defect sites. Their surfaces are more inclined to being wetted in an ethanol/CO2 system, which enables the positive metal ions to preferably absorb or localize on the defect sites. The positive metal ions can then be in situ deoxidized and decorate on the activated CNTs.50,51 This leads to a more homogeneous nucleation and formation of smaller silver nanoparticles compared with that of CNFs as the substrate. 3.4. Antibacterial Behavior of Ag Nanoparticles Wrapping Around 1D Carbon Materials. In our study, the antibacterial activities of CNF/Ag and SWCNT/Ag nanohybrids obtained under 12 MPa/65 °C were investigated. The growth rate of the bacterium E. coli was determined in a nutrient broth, and the results were determined by optical density (OD,

Figure 10. Growth inhibition by CNF/Ag nanocomposites of various concentrations against E. coli.

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Figure 11. Growth inhibition by CNT/Ag nanocomposites of various concentrations against E. coli.

absorbance) measurements. E. coli was incubated in a growth medium containing 1D carbon materials/Ag nanohybrids, and the change of optical density of the medium with incubation time was measured using a UV-vis spectrophotometer at 600 nm. Figure 10 shows the results of the CNF/Ag nanohybrids against E. coli. In Figure 10, compared with the control, no increase of absorbance with time was observed in 1% and 0.5% resultant solutions, whereas the absorbance of the medium containing 0.1% and 0.05% CNF/Ag nanohybrids increased with incubation time. The results indicate that the CNF/Ag nanohybrid possesses antibacterial activity against E. coli and the minimum inhibitory concentration (MIC) is 0.5%. Figure 11 shows the antibacterial activities of CNT/Ag nanohybrids. In the case of CNT/Ag nanohybrids, it can be seen that the optical density of the control and the bacterial suspensions of 0.1% and 0.05% CNT/Ag nanohybrids were high in comparison with that of the bacterial suspensions with 1% and 0.5% silver-loaded CNTs. This also depicts that, with time, the growth of E. coli cells is inhibited in the nutrient broth containing 1% and 0.5% silver-loaded CNTs. For quantitative measurements, the spread plate method was adopted. The nutrient agar was spread onto a Petri plate, and E. coli culture was spread onto it. Figure 12 shows the formation of bacterial colonies on the control and silver-loaded CNF after 4 h of cultivation. Compared with the control, a marked difference was observed in the plates containing 1% and 0.5% CNT/Ag nanohybrids (Figure 12) in that the growth of bacterial colonies was almost prevented. This confirms that silver nanohybrids can efficiently inhibit the growth of E. coli. 4. Conclusion In conclusion, silver nanoparticles wrapping around 1D carbon materials have been synthesized by an environmentally benign and simple method with the assistance of SC CO2. In this method, AgNO3 was used as the salt precursor and glucose was used as the reducing agent. The 1D carbon materials, CNFs and CNTs, are used as the substrate for Ag nanoparticles to grow along their axes. The TEM and SEM images show that CNT/Ag nanohybrids possess a preferable assembled structure. UV-vis spectroscopy reveals that AgNPs coating on 1D carbon materials have a relatively good polydispersity and AgNPs adsorbed on CNTs are smaller than that on CNFs. Different growth mechanisms of Ag nanoparticles on CNFs and CNTs

Figure 12. Inhibitory effects of various concentrations of CNT/Ag nanocomposites against E. coli after 4 h of incubation.

are proposed. Experimental results demonstrated that the prepared 1D carbon nanomaterials wrapped by silver nanoparticles can exhibit good antibacterial activity against E. coli and inhibit the bacterial growth at their concentration higher than 0.5%. Acknowledgment. We are grateful to the National Natural Science Foundation of China (Nos. 20974102, 50955010, and 20804040) and the Natural Science Foundation of Henan (Nos. 092300410048 and 092102310051) and for the financial support from the Ministry of Personnel and the Program for New Century Excellent Talents in Universities (NCET). References and Notes (1) Gao, C.; Li, W. W.; Jin, Y. Z.; Hao Kong, H. Facile and LargeScale Synthesis and Characterization of Carbon Nanotube/Silver Nanocrystal Nanohybrids. Nanotechnology 2006, 17, 2882–2890. (2) Chatenet, M.; Micoud, F.; Roche, I.; Chainet, E. Kinetics of Sodium Borohydride Direct Oxidation and Oxygen Reduction in Sodium Hydroxide Electrolyte: Part I. BH4- Electro-Oxidation on Au and Ag Catalysts. Electrochim. Acta 2006, 51, 5459-5467. (3) Isse, A. A.; Gottardello, S.; Maccato, C.; Gennaro, A. Silver Nanoparticles Deposited on Glassy Carbon. Electrocatalytic Activity for Reduction of Benzyl Chloride. Electrochem. Commun. 2006, 8, 1707–1712. (4) Ren, X. L.; Tang, F. Q. Enhancement Effect of Ag-Au Nanoparticles on Glucose Biosensor Sensitivity. Acta Chim. Sin. 2002, 60, 393. (5) Voccia, S.; Ignatova, M.; Je´roˆme, R.; Je´roˆme, C. Design of Antibacterial Surfaces by a Combination of Electrochemistry and Controlled Radical Polymerization. Langmuir 2006, 22, 8607–8613. (6) Li, Z.; Lee, D.; Sheng, X.; Cohen, R. E.; Rubner, M. F. TwoLevel Antibacterial Coating with Both Release-Killing and Contact-Killing Capabilities. Langmuir 2006, 22, 9820–9823.

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